Millions of women using a common contraceptive pill could be at greater risk of developing a brain tumour, new research suggests.

The mini-pill—a daily birth control tablet that contains the hormone progesterone—is often prescribed as an alternative to the standard combined pill, which contains both progesterone and another hormone, oestrogen.

But scientists have now found that women who take it for more than five years are more likely to develop a brain tumour known as a meningioma.

Although usually non-cancerous, these tumours can still be dangerous as they grow, pressing on the brain and causing symptoms such as headaches, seizures, vision problems and memory loss.

The warning comes just weeks after it emerged that hundreds of women in the UK are considering legal action over fears that a contraceptive injection containing similar hormones may pose the same risk.

A separate study earlier this year found that women who had the jab for more than a year were five times more likely to develop a meningioma.

In the latest research, French scientists analysed medical records from over 8,000 women who had surgery for the condition. 

They found many had been taking a mini-pill containing the active ingredient desogestrel—a type of progesterone—for five years or longer.

However, health experts urged women taking the mini-pill  not be alarmed, as the overall risk remains very low.

Desogestrel is often prescribed to women who cannot take oestrogen-containing contraception due to health reasons or side effects.

Although the study did find a link between long-term use and meningioma, the researchers, writing in the British Medical Journal, stressed that the increased risk was small.

They estimated that just one in every 67,000 women taking desogestrel would develop a meningioma serious enough to require surgery.

The increased risk was only seen in those who had taken the mini-pill for five years or more—and disappeared within a year of stopping treatment.

Meningiomas are tumours that form in the lining of the brain. More than 90 per cent are not cancerous, but they can still cause serious problems as they grow.

Every year proportion of patients who suffer one will need surgery to remove the tumour due to it causing increased internal pressure on the brain.

Common symptoms of a meningioma include a persistent headache as well as constant nausea which is often accompanied with drowsiness.

While not at risk of spreading elsewhere in the body like a cancerous growth they can still cause patients problems.

Dr Mangesh Thorat, an expert in population health at Queen Mary University of London, who was not involved in the study, said: 'The magnitude of increase in the risk [of menigioma] is small.' 

'Short-term use [of the mini-pill] is not associated with increased risk and that the excess risk ceases to exist once the use is stopped for more than a year.' 

He advised women who may be concerned about their risk to contact their GP.

Experts also pointed out a key limitation of the study—the researchers only looked at one type of mini-pill, desogestrel, and did not assess other progesterone formulations.

Dr Jyoti Thorat, from the Institute of Cancer Research in London, said the findings could not be applied to all versions of the drug, particularly those not commonly prescribed in France. He has called for further studies to be carried out in other countries to assess the risk more widely.

In the UK, around six per cent of women aged 16 to 49—roughly 3.1 million—currently use the mini-pill.

Taken daily, the pill works by thickening cervical mucus and thinning the womb lining, making it harder for sperm to reach an egg or for a fertilised egg to implant. In some women, it also prevents ovulation.

When used perfectly, the mini-pill is over 99 per cent effective. But with typical use—such as missing a pill or being unwell—around one in ten women may become pregnant.

Side effects can include nausea, headaches, mood swings and breast tenderness. Some users also report weight gain, though the NHS says there is no evidence the pill causes this.

The latest study follows research published last month which found that women with asthma who take the mini-pill may be a third more likely to suffer an attack.